How FRP composite materials result in an efficient tidal device?
Tidal Energy Special
A company like Gurit categorically states that Fibre Reinforced Plastic (FRP) composite is the logical material choice for most sub-sea devices, and tidal power generators fall firmly within this category.
Gurit is already witnessing a trend for components of tidal power generators, previously manufactured from steel, to be re-engineered in composites primarily for corrosion and weight reduction reasons.
"We believe that as this fledgling industry becomes more familiar with the properties and advantages that composites offer, this trend will only grow," says Thomas Royle, commercial development manager, Gurit.
"A significant project Gurit has been working on over the last 12 months is the first all-composite tidal device we have seen, and partly through its novel design, which includes the use of composites throughout, this demonstrates clear advantages over other technologies."
Royle, in an interview with tidaltoday.com, spoke about constructing fibre glass tidal devices, the strength of FRP tidal device, the major challenge for both the composites and tidal industries etc. Excerpts from an interview:
tidaltoday.com: Can you provide an insight into latest developments related to constructing fibre glass tidal
devices? How do you assess their current efficiency levels?
Thomas Royle: Gurit has extensive experience in the structural engineering of large structures over a wide range
of applications. We have engineered more large structures exclusively using composite materials than any other company and typically complete between 100 and 200 projects per annum, mainly for the marine environment. These include a number of demanding subsea structures and a tidal power generator.
Gurit has also engineered wind turbine blades up to 60m in length however blades for subsea applications are subject to far greater structural challenges with higher loads and greater risk of fatigue. A number of patents have been lodged by people who believe that they have the best blade design concept.
The question of efficiency is an interesting one. How is this measured? What we do know is that the initial build cost of a tidal or wave energy device is not the only issue. The costs associated with installation and subsequent maintenance are of far greater significance. What is required is a device capable of being installed at low cost, and which will operate maintenance free for a long period. This requires a structure free of corrosion and thus subsequent mechanical failure. One which is as light as possible to allow the use of the lowest possible vessel for load-out and one which is neutrally buoyant to further reduce installation costs. FRP composite materials provide all
of the above and thus contribute to a very efficient device.
tidaltoday.com: What according to you is the strength of Fibre Reinforced Plastic (FRP) tidal device? For instance, in case of Tidal barrages, they are expensive to build but are cheap to operate, although there can be problems with silting behind the barrage.
Thomas Royle: A major advantage of a FRP (Carbon or glass fibre) composite manufacturing process is that once
the tooling is made, subsequent units can be rolled out in quick succession but to identical and very high tolerances. This means that construction of tidal power turbines for an array or tidal farm can be realised much more quickly and at a cost which is very close to, or lower than the equivalent, of structures fabricated from conventional materials. The downside is that the tooling cost is an upfront expenditure and may therefore deter use for 'commercial' prototypes where the complete tooling cost is not spread over a number of devices.
In comparison to a tidal barrage an equivalent scale tidal farm comprised of composite turbines would be:
- As cheap to build to a fabricated structure- Less vulnerable to extreme weather conditions
- Could be rolled out progressively therefore generating electricity many years before a tidal barrage would be complete
- Naturally highly resistant to corrosion and with simpler anti-fouling technologies a well designed tidal turbine should have lower or equally low maintenance requirements
- Any tidal farm would have a much smaller environmental footprint, less silting, minimal destruction of habitat, minimal alteration of sediment flows, no obstruction to marine life passage (or to navigable vessels)
- Much lower CO2 emissions and CO2 payback period.
By the end of roll out period an FRP tidal farm would already have a highly positive CO2 balance, whereas a tidal barrage would have an enormous mountain of CO2emissions built up, which it would have to chip away at for years before it actually contributed to lowering world wide CO2levels.
However, tidal barrages and tidal farms operate in different water depths and environments, so its not really a question of one or the other. It's likely that we will have both? One of our main concerns though is that government may only be prepared to support one technology, and in that case they may choose the BIG project, (i.e. a barrage) because that is what they understand and it would generate maximum publicity and PR.
tidaltoday.com: What options, according to you, are available for building devices that are easier to install, operate, maintain and resist corrosion?
Thomas Royle: FRP composite construction accords a highly corrosion resistant finish to any structure which can incorporate a suitable anti-foul finish as has been done in the yacht building industry for years. With a relative density of 1.5 compared to 7.7 for steel the final weight of any composite device will be a quarter or less than that of a similar steel device. What this means for a tidal turbine is that a much lower rated (and therefore more common and cheaper) vessel is required for installation and maintenance operations. Onshore handling also becomes easier and safer with smaller cranes and vehicles. The potential of neutral buoyancy would allow larger devices to be towed out rather than be barged.
tidaltoday.com: Do you foresee usage of fibre glass tidal devices in the time to come? What according to
you are going to be the major challenges as far as the construction of fibre glass tidal devices is concerned?
Thomas Royle: The most obvious use for FRP composites in tidal devices will mirror the wind industry and will be
deployed in blades initially. At Gurit, we see this trend happening already as indeed is its use or proposed use for superstructures and sub-sea pods. However, there is already at least one concept that is almost exclusively composite and that is where we would expect the industry to move, over time.
Composites have many advantages over traditional materials but they have to be engineered, designed and manufactured properly, otherwise issues such as non-homogeneity, de-lamination, poor bonding etc. will arise. In the yachting industry, in the late 70's there was a move to carbon masts replacing the then default aluminium masts. Some of the new carbon masts were not designed optimally which caused some high profile failures. This in turn created the perception of a faulty material which resulted in carbon dropping off the radar. However, carbon masts continued to be developed and properly designed and have now come to dominate the market.
Gurit's concern, and the major challenge for both the composites and tidal industries in this new and exciting application, is to make sure we don't repeat those mistakes and we do engineer these devices to make the best of the materials properties and in particular to produce highly reliable structures with great longevity. The design, selection and advice on manufacturing of composite is a key skill, and one of the reasons people talk to Gurit.
In an environment where money is tight and front end engineering is expensive, this might be a difficult decision, but I believe a critical one which all participants need to make if they are to avoid major financial crises downstream.
